The term keyword density is a term used to describe the percentage of keywords on a page. Different search engines have different optimum limits, and it's wise to experiment with the frequency and placement of keywords throughout your page.

Another common term, keyword prominance; describes the position of your keywords in relation to the body of content on your page, and throughout the tags in your code.

Professional tools, such as Web CEO give an indication of keyword density (referred to as keyword weight in the program) and also keyword prominance. You want to achieve the highest prominance possible, without resorting to spam and while retaining good content. D.

Generally speaking Google, currently the most popular search engine, has an optimum percentage of around 2%, while Yahoo! Search and MSN Search are quite comfortable with a density around 5%. It's clear, that in order to achieve the best possible outcome on the big three, you need to make a compromise.

Some of the decision making is made for you when you discover the KEI of the keyword or phrase you're pursuing. More competitive keywords will require a single minded focus on a page. For instance, if your keyword has a KEI of 10 or less, then you will need to devote a page entirely to that keyword, or perhaps several pages without being repetitive. If the KEI is around 100 you may wish to add other keywords and phrases covering a variety of topics in your field.

For an outline of how your should place keywords for maximum density please visit the search engine spider page.